Thermochemistry (Chemistry for Engineers) Masterton.pptx

Thermochemistry

1. Thermochemistry Reference: CHAPTER 8 Chemistry for Engineering Students Masterton, W.L., Cengage 2018.

2. Basic Principles of Heat Flow Review of Past Lesson: System, Surrounding, Boundary 2. Types of System

3. Review of Past Lesson State of a System State Properties – depend only on the state of the system, not on the way the system reached the state. where X is a state property

4. Direction of Heat Flow (Calorimetry) When hot plate is then shut-off the hot water gives off heat to the surroundings q , for the system is a negative quantity. q is negative when heat flows out of the system from the surroundings . Set up in Figure 8.1. When hot plate is on, there is a flow of heat from the surroundings into the system. The heat flow, q , for the system is a positive quantity. q is positive when heat flows into the system from the surroundings .

5. Review of Past Lesson : Endothermic vs Exothermic Process Endothermic process (q >0), heat flows from the surroundings into the reaction system. Example: Photosynthesis; Melting of Ice. Exothermic process (q < 0), heat flows from the reaction system into the surroundings Example: Combustion

6. Magnitude of Heat Flow SI base unit of q: Joule (kilojoule) Another unit of q: calorie Calorie – the amount of heat required to raise the temperature of 1 g of water one degree Celsius. 1 cal = 4.184 J 1kcal = 4.184 kJ kcal can also appear as Cal

7. Heat Capacity, C and Specific Heat, c Heat capacity: The amount of heat required to raise the temperature of the system 1 degree Celsius. q = C x SI unit: For pure substance of certain mass: q = m c x Specific heat: the amount of heat required to raise the temp of 1 gram of a substance 1 degree Celsius.

8. Measurement of Heat Flow: Calorimetry 2 Types of calorimeter: Cup calorimeter Bomb calorimeter

9. Cup Calorimeter

10. 4.3 Bomb calorimeters can be used to determine the heat of combustion. If you test a reaction that releases heat, the temperature of the water will increase.

11. Enthalpy, H Enthalpy – a type of chemical energy, also known as “heat content”

12. Enthalpy relationship between reactants and products 1 mol of carbon dioxide and 2 mol water have lower enthalpy than the reactants, 1 mol methane and 2 mol oxygen. The decrease in enthalpy is the source of the heat evolved to the surroundings

13. Enthalpy relationship between reactants and products Liquid water has a higher enthalpy than ice, so heat must be transferred from the surroundings to melt the ice

14. In general:

15. Bond Enthalpy (Bond energy)

16. 4.4 Energy Changes at the Molecular Level The energy changes are due to the rearrangement of the atoms of the reactants and products; it is the breaking and forming of bonds that dictates if a reaction will be endothermic or exothermic. Bond energy is the amount of energy that must be absorbed to break a chemical bond. energy Breaking bonds ALWAYS requires energy!

17. 4.4